U.S. patent applications Ser. No. 181,974, filed Aug. 28, 1980 and now abandoned, Ser. No. 295,000, filed Aug. 21, 1981 and now U.S. Pat. No. 4,485,297 issued Nov. 27, 1984, Ser. No. 487,080, filed Mar. 23, 1983, now abandoned and Ser. No. 572,678, filed Jan. 20, 1984, now U.S. Pat. No. 4,523,085, issued June 11, 1985 all of which, together with Ser. Nos. 580,742, now abandoned, and 646,688, now U.S. Pat. No. 4,542,285, are owned by the assignee of the present application and are here incorporated by reference, disclose flexible sheet heaters including a pair of longitudinally-extending (typically copper) conductors, and a semi-conductor pattern extending between and electrically connected to the conductors. The heaters there disclosed provide generally superior performance and substantially even heat distribution, and are useful in a wide range of applications.
There are circumstances, however, in which the heater may be subjected to unusual or extreme conditions. For example, Ser. No. 580,472 discloses that heaters may be used as, for example, firing range targets. When used as a target for small arms, there is a very real chance that a bullet may strike and sever one of the longitudinally-extending conductors, and render the heater inoperative. Similarly, the shock of a large calibre shell, when for example the target is used on an anti-tank weapons range, may cause partial delamination or other structural damage, even when the "hit" is wholly in the area between conductors. High stresses or risk of breakage may be presented in other environments or applications; and other potential difficulties are presented by the need, in some applications, to operate using relatively high voltage or current levels.